The need for low emissions has created an increased demand for light olefins for use in alkylation, oligomerization, and MTBE and ETBE synthesis processes. In addition, a low cost supply of light olefins, particularly propylene, continues to be in demand as feedstock for polypropylene production.
Light olefins, such as ethylene and propylene, can be produced by thermally cracking naphtha feedstocks containing paraffinic and isoparaffinic compounds, naphthenes and aromatics to produce light olefins. The thermal cracking of naphtha is carried out by exposing naphtha and steam to relatively high temperatures in the tubular coils of a fired heater. A problem associated with this technique is that the process is energy intensive, not very selective, produces coke, and releases significant amounts of carbon dioxide into the air.
Another technique for producing light olefins involves the catalytic cracking of hydrocarbons, such as naphtha. In the catalytic cracking of naphtha, the process is carried out by contacting a naphtha-containing feed with a catalyst usually comprised of one or more crystalline microporous molecular sieves to selectively convert the feed into an olefin-containing mixture. Although various naphtha catalytic cracking processes have been proposed in the past, many of the processes do not produce commercially important light olefins, e.g., propylene, with sufficient selectivity or yield. Also, the processes can produce undesirable amounts of methane and aromatics as unwanted byproducts. In contrast, a practical and economic naphtha catalytic cracking process should selectively produce increased amounts of light olefins, e.g., propylene, while producing minimal amounts of methane and aromatics.
Fluid catalytic cracking (FCC) is routinely used to convert heavy hydrocarbon feedstocks to lighter products, such as gasoline and distillate range fractions. Conventional processes for catalytic cracking of heavy hydrocarbon feedstocks to gasoline and distillate fractions typically use a catalyst containing a large pore molecular sieve, such as zeolite Y, as the primary cracking component and, optionally, an intermediate pore size molecular sieve, such as ZSM-5. While FCC is an efficient process for converting heavier feed to lighter products, many times the process makes less than desirable amounts of light olefins, e.g., propylene.
The present invention provides a catalyst and process for the catalytic cracking of hydrocarbon feedstocks, e.g. naphtha and heavier hydrocarbon feedstocks, which is effective in producing enhanced yields of propylene, as compared with known processes used to crack hydrocarbons.